linux-kernel - Re: [PATCH 1/2] lockref: speculatively spin waiting for the lock to be released

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Date: Thu, 13 Jun 2024 11:43:05 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Christian Brauner <brauner@...nel.org>, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] lockref: speculatively spin waiting for the lock to
 be released

On Thu, 13 Jun 2024 at 11:13, Mateusz Guzik <mjguzik@...il.com> wrote:
>
> I would assume the rule is pretty much well known and instead an
> indicator where is what (as added in my comments) would be welcome.

Oh, the rule is well-known, but I think what is worth pointing out is
the different classes of fields, and the name[] field in particular.

This ordering was last really updated back in 2011, by commit
44a7d7a878c9 ("fs: cache optimise dentry and inode for rcu-walk"). And
it was actually somewhat intentional at the time. Quoting from that
commit:

    We also fit in 8 bytes of inline name in the first 64 bytes, so for short
    names, only 64 bytes needs to be touched to perform the lookup. We should
    get rid of the hash->prev pointer from the first 64 bytes, and fit 16 bytes
    of name in there, which will take care of 81% rather than 32% of the kernel
    tree.

but what has actually really changed - and that I didn't even realize
until I now did a 'pahole' on it, was that this was all COMPLETELY
broken by

       seqcount_spinlock_t        d_seq;

because seqcount_spinlock_t has been entirely broken and went from
being 4 bytes back when, to now being 64 bytes.

Crazy crazy.

How did that happen?

               Linus